This document relates generally to semiconductor devices, and more specifically to field effect transistor (FET) structures and methods of manufacture.
Electronic systems, including portable electronic systems, are typically powered by various combinations of power sources. Some of these power sources have included one or multiple batteries and AC wall outlets via an ac/dc converter or battery charger. Users of electronic systems expect safeguards to prevent damage to the internal electronics of the system in the event of reverse battery installation, incorrect converter or charger installation, accidental short circuiting, and other forms of inappropriate operation. In order to protect the electronic system from such damage occurring, some manufacturers of conventional switches have incorporated varying networks of switches to control the flow of power within the system. As an example, if a conventional switch system was powered from a primary battery while a secondary battery was charged, some switches were closed while other switches were open. In another mode, the switches may have been reversed. These networks of switches have typically been comprised of power metal oxide semiconductor field effect transistors (power MOSFETs). To be effective in all modes, the network of switches generally conduct and block in both directions. However, power MOSFETs can only block voltage in one direction. Thus, in order to ensure reverse current flow and reverse bias voltage are low enough to prevent damage to the system, two power MOSFETs are typically connected in series to function as one switch. The two power MOSFETs typically are used with their drains tied together so that when the gate voltage is zero, at least one of the devices blocks the voltage applied across the two transistors regardless of the polarity. A major downside to this setup is that the back-to-back power MOSFET series arrangement doubles both the on-resistance and chip area of the device thereby quadrupling the specific on-resistance of the device.
Accordingly, it is desirable to have a monolithic bidirectional switch that reduces on-resistance and is cost effective to manufacture.
For simplicity and clarity of illustration, elements in the figures are not necessarily drawn to scale, and the same reference numbers in different figures denote generally the same elements. Additionally, descriptions and details of well-known steps and elements may be omitted for simplicity of the description. As used herein current-carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor, an emitter or a collector of a bipolar transistor, or a cathode or anode of a diode, and a gate electrode means an element of the device that controls current through the device such as a gate of a MOS transistor.
Although the devices are explained herein as certain N-channel devices, a person of ordinary skill in the art understands that P-channel devices and complementary devices are also possible in accordance with the present description. For clarity of the drawings, doped regions of device structures are illustrated as having generally straight-line edges and precise angular corners. However, those skilled in the art understand that due to the diffusion and activation of dopants, the edges of doped regions are generally not straight lines and the corners are not precise angles.
Furthermore, the term “major surface” when used in conjunction with a semiconductor region or substrate means the surface of the semiconductor region or substrate that forms an interface with another material, such as a dielectric, an insulator, a conductor, or a polycrystalline semiconductor. The major surface can have a topography that changes in the x, y and z directions.
In addition, structures of the present description may embody either a cellular base design (where the body regions are a plurality of distinct and separate cellular or stripe regions) or a single base design (in which the body region is a single region formed in an elongated pattern, typically in a serpentine pattern or a central portion with connected appendages). However, one embodiment of the present description will be described as a cellular base design throughout the description for ease of understanding. It should be understood that the present disclosure encompass both a cellular base design and a single base design.